Piezoelectric crystal filters are commonly found in analog and digital radio communication devices. Typically, the same two-pole filter designs are used in both analog and digital applications, even though the radio requirements may be entirely different. Analog radio devices generally require filters to have a number of poles depending on the frequency response requirements of the radio. In the design of an analog radio, for example, the sensitivity and selectivity are specified and the designer incorporates filters having the requisite number of poles to provide the specified sensitivity and selectivity. If, for example, a two-pole filter will not provide the required selectivity, a four-pole monolithic crystal filter or two or more cascaded two-pole monolithic crystal filters may be used.
Digital radio devices generally require that filters provide a uniform phase response depending on the bit error rate (BER) requirements of the radio. In the design of a digital radio, for example, the group delay (change in phase over frequency) is specified and the designer incorporates predetermined discrete interstage coupling components or digital equalization to provide the specified group delay.
The design of discrete element filters of three or more poles and having uniform phase response for digital transmission are well known in the art and include Bessel, Gaussian, linear phase, equiripple, and others. However, multi-pole uniform phase response filters incorporating piezoelectric crystals in their design have been restricted to the use of single resonators or cascaded two-pole monolithic filters of a symmetric acoustic design.
FIG. 1 shows a prior art four-pole linear phase filter incorporating dual two-pole filters 10,12. The cascaded two-pole monolithic filters 10,12 are electrically coupled by asymmetric impedance matching networks to meet a phase response requirement. Generally, an input network includes a series connected capacitor 14 and a shunt connected inductor 22 and variable capacitor 18. An output network may include a series connected capacitor 16 and a shunt connected inductor 24 and variable capacitor 20. In order to provide a uniform phase response the components of the input network 14,18,22 are of different values and provide a different impedance match than the components of the output network 16,20,24. Further, the prior art design requires the use of interstage impedance matching components that typically include a shunt inductor 28 and variable capacitor 26. The interstage components 26,28 disadvantageously increase the cost and complexity of the filter and are very sensitive to tuning. In addition, individual variations between the filters 10,12 generally require the use of higher cost adjustable components 18,20,26 to allow tuning of the circuit to provide an appropriate frequency and phase response.
Radio designers have the option of using digital phase equalization of existing Tchebychev or Butterworth crystal filters or even providing filtering entirely in the digital domain without crystal filters. However, both of these options have the disadvantage of requiring the addition of an digital signal processing integrated circuit in the radio.
There is a need for a monolithic crystal filter having three or more poles and providing an improved phase response without the use of discrete interstage components or digital signal processing. Accordingly, an asymmetrically acoustically coupled multi-pole monolithic crystal filter would be an improvement over the prior art.